The present invention concerns a fuel cell arrangement consisting of several fuel cells, arranged at least essentially in parallel, with cooling gaps formed between neighboring cells extending between an inlet and an outlet and through which a coolant flows.
Low-temperature fuel cells (PEMFC) transform chemical energy directly into electrical energy. Hydrogen is used as the fuel gas. The oxidant is pure oxygen or the oxygen contained in the air. Individual cells are wired together mechanically and electrically into stacks.
Fuel cells have a load-dependent efficiency of about 50%. The heat losses must be carried off by a corresponding cooling system. In most cases this is accomplished by means of water circulation with an external cooler. Air-cooled cells or stacks are also known. Because of the absence of water pumps, water coolers, etc. such a system is clearly simpler than a comparable water-cooled system. The cells have dimensions typically of 10xc3x9710 cm2 to 30xc3x9730 cm2. The thickness of the cells is 1 to 2.5 mm. The air gap or cooling gap through which the coolant, usually in the form of air, flow is ca. 1-4 mm wide.
Analysis of the existing technology of air-cooled PEMFC stacks has shown that a significant temperature difference prevails in the membrane-electrode assembly (MEA). This temperature difference leads to a nonuniform current density distribution inside the cell with a reduced possible maximum power of the cells as a result.
Typically 50-100 kW electric power are required for use in automobiles. Stack sizes of 100 to 400 cells are derived on the basis of this requirement and depending on the size and power density (ca. 0.6 W/cm2) of the individual cells.
For cooling the stack it is necessary to blow air through the cooling gaps with a blower. Both the outsides of the cells as well as the spaces between the cells which are distributed uniformly over the cell area in the form of cooling fins serve as the heat transfer surface. The necessary power of the cooling blower for cooling a 50 kW stack should not exceed an order of 1-2 kW. Greater capacities would too strongly reduce the efficiency of the entire system in the automobile. Experience has shown that the cooling air is clearly heated up inside the stack under these conditions. From this, there results a distinct temperature difference in the MEA between the cooling air inlet and the cooling air outlet of the same order of magnitude, e.g. of about 30xc2x0 C. From this, because of the temperature dependence of the reactions taking place in the cell and the transport phenomena (electrochemistry, electrocatalysis, evaporation, condensation, material transport of gases and liquids in porous media in channels, etc.), the current density distribution inside the individual cells becomes inhomogeneous. In the case of a required average current density per cell this may lead locally to very high current densities which can cause destruction of the cell.
The objective of the present invention is to design the cooling system of the cells in such a way that the temperature difference in the MEA is clearly smaller than previously, e.g., is reduced to 10xc2x0 C. or less, in which case the cooling air despite this may have a temperature difference of e.g., 30xc2x0 C.
To solve this problem according to the invention in a first variant, a fuel cell arrangement of the type described initially is envisioned in which the specific surface, i.e., the area of the cooling surfaces emitting heat to the coolant increases in the direction from the inlet to the outlet.
In a second variation of the solution according to the invention, a fuel cell arrangement of the type described initially is envisioned, which is characterized by the fact that the local heat transfer coefficient of the cooling surfaces emitting heat to the coolant increases in the direction of flow from the inlet to the outlet.
In a third variant of the solution according to the invention, also in the case of a fuel cell arrangement of the type described initially, it is envisioned that the carrier materials of the fuel cells, i.e., the membrane electrode assemblies, have a thermal conductivity above 200 W/(mxc2x7K), a thermal conductivity which is reached, for example, by aluminum or aluminum alloys.
In a fourth variant of the invention, the cooling gaps can be designed to produce flow gradients, e.g. as a result of built-in structural elements, for the purpose of increasing the local holding time along the cooling gaps so that heat removal constantly increases.
These four solutions or variants according to the invention can also be combined with each other advantageously.
Although it would be most favorable according to the invention if the specific surface and/or the local heat transfer coefficient of the cooling surfaces emitting heat to the coolant increased continuously from the inlet to the outlet, a good approximation of the desired result can be achieved if the cooling gaps are subdivided into several regions in the direction of flow from the inlet to the outlet, with the specific surface or the coefficient o thermal conductivity exceeding in each region the specific surface or coefficient of thermal conductivity in the previous region. A subdivision into two regions would already bring about a significant improvement compared to the state of the art. A subdivision into three regions, however, is preferred according to the invention, and subdivision into four or more regions could also be considered if the length of the cooling gaps in the flow direction permitted such a subdivision.
The starting point of the invention was the question of how an ideal approximately isothermal operating state of the MEA can be realized with a sufficient and homogeneous supply of oxygen and hydrogen. Starting with this ideal operating state of the MEA, a constant current density distribution and accordingly also a constant heat loss per unit area would be established which is passed on to the air-cooled solid (support material) in which the MEA is embedded.
The convective heat transfer from the solid to the air and the heat conduction inside the carrier medium can now be configured according to the invention by the corresponding cooling fin arrangement including the choice of material in such a way that in good approximation a homogeneous heat loss flux is established from the MEA to the support material.
The invention has succeeded, as explained below, in reducing the maximum temperature of the MEA by 25% and the temperature gradient by 61% when the first variant of the invention is used. By changing the coefficient of thermal conductivity corresponding to the second variant of the invention, a decrease in the maximum temperature of the MEA by about 6% and a reduction of the temperature gradient by about 13% can be achieved. If aluminum is used as the support material in the fuel cell with a coefficient of thermal conductivity of 236 W/mK than a reduction of the maximum temperature of the MEA by 5% and a distinct reduction of the temperature gradient can be achieved. If one combines all three of these possibilities envisioned according to the invention, than a significantly more efficient cooling can be achieved so that the required driving power of the blower required can be clearly reduced as can the structural size, the weight and the cost of the cooling system.
On the other hand, it is assured that the current density distribution inside the fuel cells is significantly more uniform and can be operated closer to the maximum limit without the occurrence of premature failure of the fuel cells due to elevated current densities at certain unpredictable places. On the whole, the fuel cells can b built smaller for a specific power, which saves weight, space and cost.
Preferred variants of the invention are described in the subclaims.